The present invention relates generally to gas mixing valves and methods for mixing together two or more gases in selected proportions, and is particularly concerned with a gas mixing valve or apparatus for use with a medical respirator in order to mix air and oxygen in a desired ratio.
Gas mixing valves are used to mix two or more different gases in desired proportions and to provide a desired output gas mixture. Such valves are used in medical respirators to mix air and oxygen to provide a suitable breathing mixture to a patient. Normally, a pair of poppet valve members are used to adjust the size of two orifices for controlling the proportions of the two different gases to be mixed. The size of the orifices is adjusted according to the desired mixture. One problem with such an arrangement is that mixing accuracy may be reduced as a result of reduction in the gas flow rates. Pressure drop across each valve will be proportional to the flow rate, and will increase as flow rate increases and decrease as the flow rate decreases. Generally, gas mixing valves have relatively good mixing accuracy at high flow rates, but significant errors in mixing accuracy can arise at low flow rates.
In the past, this problem has been dealt with by designing special mixing valves for low flow rate applications, which have smaller scale valves. This limits the range of flow rates over which any one mixing valve can operate effectively. Another solution is described in U.S. Pat. No. 4,072,148 of Munson et al., in which the mixing valve is provided with two stages. One, smaller valve stage is operated at all times, regardless of flow rate. The other, larger valve stage is operated only at high flow rates. Another alternative mixing valve arrangement is described in U.S. Pat. No. 4,085,766 of Weigl et al. In this apparatus, a piston is slidable in response to change in a reference gas pressure in order to adjust the size of two gas orifices in a sleeve surrounding the piston.